Signaling system



3 Sheets-Sheet l Emu 33v /N V5 N To l? P. MERTZ P. MERTZ SIGNALINGSYSTEM ZEN/7 Filed Aug. 10, 1940 Jan. .5, 1943.

Jan. 5, 1943.

P. MERTZ SIGNALING SYSTEM Filed Aug. 10, 1940 3 Sheets-Sheet 2 INVENTORP MER TZ ATTO NEV Jan. 5, 1943. P. MERTZ 2,307,728

SIGNALING SYSTEM Filed Aug. 10, 1940 3 Sheets-Sheet 3 (2000 CPS) .cPs)

L INVENTOR l P. MERT Z LINE I l B) B m A Patented Jan. 5, 1943 UNITEDSTATES PATENT. OFFICE SIGNALING SYSTEM Pierre Mertz, Bellerose, N. Y.,assignmto Bell Telephone Laboratories, Incorporated, New York, N. Y., acorporation of New York Application August 10, 1940, Serial No. 352,04128 Claims. (Ci. I'm-6.6)

This invention relates to signaling and particularly to systems for theproduction and transmission o'f image signals and the reconstitution ofa picture from such signals at a distant point.

A principal object of the invention is to effect economies in 'thefrequency space and time required for the transmission of image signals.

Other objects are to provide improved apparatus and methods for theproduction of image signals and the reconstitution of pictures fromimage signals with a minimum of distortion.

Attention has already been given to the problem of increasing the speedof image signal transmission and therefore reducing the total timerequired, and various systems directed to the solution of this pro lemare known. For example, Patent No. 2,301, 99 toE. Bruce and W. S. Gortongranted Nov. 10, 1942 on an application filed April 26, 1940, describesand claims a. signaling system in which scanning at both transmitter andreceiver takes place at high speed for white areas of the picture and atnormal speed for dark areas, thus effecting an increase in speed and a Isaving of transmission time as compared with the speed and time whichwould be required if the entire process were carried out at normalspeed.

In the picture or image signaling art, the frequency space required isgoverned mainly by the sharpness of the discontinuities or boundariesbetween light areas and dark areas and the perfection which is requiredin the reproduction of these areas. In any system employing variablespeed scanning this limitation is complicated by the fact that thetransmission channel must be capable of conveying information to thereceiver as to the precise instant at which it. should accelerate ordecelerate as each boundary between a dark area and .a light area isreached or vice versa. The system of the above-mentioned Patent No.

i 2,301,199, for example, requires for proper operation a frequency bandgreat enough to carry this important speed change information with such2 precision that substantially instantaneous response of the receiverapparatus is assured. Thus that system, though it accomplishes anincrease in speed and a'saving in time, does not, without furthermodification, effect any reduction in the required frequency space overthat required by conventional systems. It turns out, however, that thatsystem lends itself easily to modification whereby a substantial savingin frequency space is achieved.

Accordingly, a particlular object of this invention is to providemodifications of variable spee scanning systems which shalleffectsubstantial economies of frequency space without loss of speed ordetail.

The problem of conveying speed change information from a picture signaltransmitter to a receiver of the variable speed scanning type hasalready been recognized, and it has been proposed to provide thetransmitter apparatus with an auxiliary scanner, similar to the mainscanner and placed in advance thereof, which shall derive from thepicture itself a sequence of picture si nals in advance of the mainpicture signals and identical therewith, except in so far as they areadvanced in time with respect thereto. These advance signals constituteadvance information as to the character of that part of the picturewhich at any instant is about to be scanned by the principal scanner,either at transmitter or at receiver. It has been proposed to utilizethem to control the speed of a motor which determines the scanningspeed. This proposal is open to the serious objection that, with twolike scanners and no additional discriminating apparatus, theinformation delivered to the principal scanner by this premonitorysignal is pust as likely to be wrong as it is to be right, causing thescanning to accelerate when it should decelerate or decelerate when itshould accelerate, thus defeating the very purpose which it was designedto serve.

In contradistinction to this proposal and in furtherance of the objectsof this invention, there is provided in accordance wtih the invention infor correctly governing the scanning speed both at transmitter and atreceiver. This premonitory signal, comprising only two fairly narrowfrequency bands, may then be transmitted to the receiver to control theacceleration and deceleration of the printer apparatus. At the same timethe principal picture signals may be condensed into a single very narrowfrequency band and may be transmitted to the receiver over a separatecommunication channel. The end result of this system is that theinformation-carrying frequency band, instead of extending all the wayfrom a very low frequency to the highest frequency required to conveythe information as to boundaries between black and white areas,consists, in the black and white system, of only three narrow frequencybands, two of them transmitted on one channel and one on another.

In accordance with another modification of the invention, still furthersaving or economy in frequency space may be achieved by entirelyeliminating the premonitory signal transmission channel and deriving theequivalent information from the main picture signal itself at thereceiver. The frequency space required for transmission of all thenecessary information consists, in this modification, solely of a singlenarrow band. This result is accomplished, in accordance with this secondmodification of the invention, by the provision of local generators atthe receiver station, one being a generator of white scanning"oscillations and another a generator of black scanning" oscillations,either one of which, but not both, governs the scanning operation in thereproducer and causes it to progress at a speed related to thetransmitter scanning speed. The signal received from the transmitter,which may be a sole blacld signal, the white" component having beensuppressed at the transmitter, operates to switch in the black generatorwhen it is coming through and to switch it off and switch in the "whitegenerator instead when the signal is not coming through. The receivedsole black" signal may also operate or control the printing operation toeffect printing of dark areas only when the sole black signal is comingthrough. Thus a feature of this modification is the operation of thereproducer for both black and white printing through the medium of asole black received signal alone.

In accordance with a preferred form of this modification, the signalcharacteristic which is related to the characteristic of the picture isits frequency. Thus the sole black signal is a low frequency signal andthe suppressed white signal is a higher frequency signal. The localblack signal generator is of a comparatively low frequency-and the localwhite generator is of a higher frequency.

In order to assure that the printer scanning shall be taking place atslow speed throughout the time that printing is in progress and that itshall not accelerate unless a considerable white area appears ahead ofit, there is preferably provided in addition a temporary storage devicefor the low frequenc sole black signal from which, in turn, bothprinting signals and scanning speed signals are thereafter derived. Inaccordance with a preferred form, the incoming sole black signals may berecorded on a magnetic tape or the like and subsequently recovered fromthe tape by two pick-up devices, the first one being of comparativelylarge aperture and the second one being of normal picture elementaperture. Premonitory signals are derived from the first pickup deviceand main scanning signals from the second. The premonitory signals arethen utilized to'actuate the scanning of the reproducer and the mainsignals to actuate its printing device. This results in the preservationof definiteness and precision of the operation of the reproducer despitethe fact that when the communication channel is very narrow the build-upof the main sole black signal is necessarily slow. This modificationtherefore permits the insertion a; a narrow band-pass filter in thetransmission The invention in its many possible forms will be more fullyunderstood from the following detailed description of preferredembodiments primarily suited to the transmission and reconstitution ofblack and white pictures or copy taken in conjunction with the appendeddrawings. It will be understood that the terms picture, "copy" and thelike are herein employed not to distinguish typed or printed materialfrom representations of scenes, portraits, and the like, but rather toemphasize their equivalency for the pur-. poses of the invention. In thedrawings:

Fig. 1 is a circuit diagram of a transmitter system illustrative of oneembodiment of the invention;

Fig. 2 is a highly schematic diagram of a scanning device in accordancewith the invention;

Fig. 3 is a circuit diagram of receiver apparatus adapted to respond tothe signals delivered by the transmitter apparatus of Fig. 1;

Fig. 4 is a circuit diagram of receiver apparatus illustrative of amodification of the invention; and

Fig. 5 illustrates the arrangement of parts of the apparatus of Fig. 4to an enlarged scale.

Referring now to the figures, the picture or other copy 1 to betransmitted may be mounted or supported on a belt or tape 2 which passesover rollers 3, 4 one of which is provided with a ratchet and pawlmechanism 5, 8 for intermittent line-by-line advance thereof. Light froma long line filament l is sharply focussed through a lens 8 onto asingle line of the copy I. Light reflected from this illuminated copyline is focussed by another lens 9 as a line image ill on thephotoelectric cathode surface Ii of a double-aperture scanning device12.

The scanning device which is preferred though not required for use inconnection with the signaling system of this invention is shown ingreater detail but still schematically in Fig. 2, It may constitute amodification of apparatus known per se and commonly designated by theterm image dissector tube. As shown in Fig. 2, it comprises a glassenvelope l3 provided at one end with a photoelectric surface II whichplays the part, in operation, of a cathode. A screen 15 of wire mesh isplaced in front of the cathode and serves as an accelerating anode forthe beam I! of electrons emitted from the photoelectric cathode II. Abattery or other source of potential I6 is connected between the cathodeII and the screen H in well-known manner in order to give the electronsa substantial initial velocity axially of the tube.

The tube may be provided with a magnetic iocussing coil to be energizedwith direct current in well-known manner and also with a single pair ofmagnetic beam-deflecting coils. Focussing coil l1 and deflecting coilsit are schematically indicated in Fig. 2, but in the interests ofsimplicity, source and connections have been omitted in each case. For amore complete description of the construction and mode of operation ofthe image dissector tube, reference may be made to Farnsworth Patent1,773,980 and to the Journal of the Franklin Institute, vol. 218, pages411-444.

As customarily constructed, dissector tubes are provided at the endremoved from the cathode end with a plate I having therein a smallaperture 20 behind which is placed the electron-collecting anode 22which is connected through a a modification of major importance is theprovision in the plate i4 and adjacent the aperture 20 of .an auxiliaryaperture 2| of several times the size of the ordinary aperture, and acooperating auxiliary anode 23 likewise connected through an auxiliaryloading resistor 25 to the high potential source i6. Since it iscontemplated that scanning shall take place only in one direction,motion in the other direction being secured by movement of thecopy-supporting tape 2, the photoelectric part of the cathode mayconsist of a relatively narrow line ofphotoelectric material instead ofa large area. This line emits electrons in the form of a sheet ratherthan a bundle. The electrons are directed axially of the tube by thefocussing coil I1 and, when the tube is in operation, are deflected fromside to side in the direction in which the two apertures 20, 2| lie bythe magnetic fields of the beam-deflecting coils l8.

In order to economize space and reduce the stresses due to atmosphericpressure the tube may be much narrower in the direction perpendicular tothe scanning direction than in the scanning direction and it is so shownin Fig. 2.

Obviously, a dissector tube of ordinary construction, that is, with aphotoelectric cathodecovering a substantial area and two pairs ofdeflecting coils may be employed, the tube being modified by theprovision as described above of the two apertures and two associatedanodes. Again, a cylindrical tube may be provided with a line cathode,or tube of any form may be provided with two pairs of deflectingelements. The deflecting elements may, if desired, be plates whichoperate by electrostatic deflection, although magnetic deflection isusually more suitable for apparatus of this type. Any or all of theabove, or other, variations may be employed provided the tube has theessential feature of two apertures with a separate collector anodeassociated with each one, or its equivalent.

Returning now to a consideration of Fig. 1, an.

optical image l0 of a line of copy is formed, as above described, on thephotoelectric cathode ii of the dissector tube I2. An electron beam I!in the form of sheet is produced at the cathode II and accelerated bythe screen i5 and travels down the tube toward the apertures to form anelectron image of the illuminated copy line I! in the plane of theapertures 20, 2| in a manner well known per se. This beam is deflectedback and forth in the embodiment shown by the magnetic field due to adeflection current in the beamdefiecting coils 18. These deflectingcoils i8 are schematically indicated above and below the dissector tubel2 and in the plane of the electron sheet. As is well known, in actualpractice these coils should be placed centrally above and below theelectron sheet in order to sweep the sheet in its own plane. Placementand design of the defleeting coils form no part of the invention andaccordingly the drawings are simplified by showing the coils in highlyschematic form and disposition.

The circuit arrangement for delivering current of appropriate wave formto the deflecting coils ll, both to sweep the electron beam l9 past theapertures Z0, 2! in the scanning direction and to return it rapidly toits starting point, will be fully described hereinafter. For the presentit will sufflce to state that the deflection is such that the electronbeam i9 sweeps past the apertures in a direction which is downward inFig.2 and at a speed which varies instantaneously in accordance with thebrightness or degree of light and shade of the elementary areas of copybeing scanned. For example, the white speed may be ten times or so asgreat as the black speed.

Electrons which enter the principal aperture 20 strike the principalanode 22 and electrons which enter the auxiliary aperture 2| strike theauxiliary anode 23. In accordance with the invention the principalaperture 20 is preferably of a size corresponding to the size of anelement of the picture or copy being scanned. Apart from either opticalor electronic magnification it may be equal in size to a pictureelement. Either optical magnification or electronic magnification orboth may be employed with the result that the electron image in theplane of the apertures will depart considerably in size from the copybeing scanned, in which case the size of the prin-- cipal scanningaperture 20 may be adjusted accordingly.

The auxiliary scanning aperture 2! is placed as close as possible to theprincipal scanning aperture 20 and in the same electron image plane. Asabove stated, it is considerably larger than the principal aperturebeing in the preferred embodiment fifteen to twenty times as long. Bothapertures, however, are of the same width in the direction perpendicularto the scanning direction,

each being preferably as wide as the electron image ll of a singlescanning line of copy.

In the figures the main and auxiliary apertures have been muchexaggerated in size in order to bring out their relative sizes andseparation. As actually constructed, the main aperture may be about 0.01inch square and the auxiliary aperture about 0.2 inch long by 0.01 inchwide, the

- separation between them being about 0.01 inch in length.

From this construction it will be apparent that the electrons which, atany instant, enter the aperture 20 and therefore constitutes an advancesignal.

The lower part of Fig. 1 shows a circuit arrangement for utilizing theseadvance signals in order to attain the object of the invention.

The principal anode 22 and the auxiliary anode 23 of the dissector tubeI2 are respectively connected through individual loading resistors 24,25 to the positive terminal of a source of potential such as a batteryl6 whose negative terminal is connected to the cathode Ii. The grid i!and the apertured plate i4 are connected to a point of suitablepotential, such as a tap of the battery i8. Currents due to electronsstriking the main anode 21hr the auxiliary anode 23 respectivelytherefore give rise to voltage drops across the loading resistors 24,25. The input circuit of a two-stage amplifier K which may be of anysuitable construction and is represented in the figures as a simpletandem arrangement of triode circuits is connected across the principalanode loading resistor 24 and the input circuit cl 9. similar amplifierJ is connected across the auxiliary anode loading resistor 25. As iswell known, each stage of each of these amplifiers constitutes a phasereversing device and consequently voltages at the output resistor I4 ofthe first stage of the amplifier K, due to a given input signal, will beof opposite phase to voltages at the output resistor 36 of the secondstage due to the same signal. The same will be equally true of the phaseconditions in the amplifier J.

The first stage of the amplifier J is preferably biased, for example, bya C battery I I, well below its cut-f! so that while full current in thecircuit of the auxiliary anode 23 serves to make it conductive, 95 percent full current does not. The amplifier K on the other hand. ispreferably biased only slightly below cut-oil, for example by C battery30. The reasons for these dlflerences will appear more fullyhereinafter.

Though not in any way essential to the invention, discrimination betweenwhite copy areas and black copy areas on the basis of change in signalfrequency offers conveniences in carrying out the objects, andaccordingly, in illustration of this preferred embodiment afrequency-modulated signal is employed, the tone values of the pictureto be transmitted being associated with diiIerences in the signalfrequency rather than differences in the signal amplitude or othersignal characteristic. In the particular case of twotone orblack-and-white copy, the frequencymodulated signal reduces tooscillations of one frequency corresponding to dark picture areas andoscillations of the same amplitude but of a difierent frequency to lightpicture areas. Preferably, the higher frequency corresponds to thelighter areas.

The remainder of the apparatus of Fig. l is constructed on the basis ofthis frequency discrimination. A generator 4| which may be of anydesired type, for example, a vacuum tub oscillator whose connections arewell known per se, supplies energy of a higher frequency, for example2000 cycles per second, to the input. circuit of a discharge device, forexample vacuum tube 42. As shown, the terminals of the generator 4| areconnected through a stopping condenser 42 to the cathode 44 and to thefirst control grid 45 of the tube 42, the grid being returned through aresistor 46 and a biasing battery 41 to the cathode 44. The plate oranode 48 of this tube 42 is connected to a plate loading resistor 48 andan anode voltage supply battery 50 to the cathode 44. With theinterposition of a stopping condenser i and a band-pass filter 52, theoutput circuit of the vacuum tube 42 is connected to a transmission line53 designated as "line I". The filter 52 may be of any desired type andmay be constructed to pass only a relatively narrow band of frequenciescentered in the neighborhood of 2000 cycles per second.

Another generator or oscillator ii of a considerably lower frequency,for example 200 cycles per second, supplies the input circuit of anotherdischarge device or vacuum tube 62 whose output circuit is likewiseconnected, with the interposition of another band-pass filter 12, toline I. The input and output circuits of the second tube 62 and theirconnections to the generator and the filter may be exactly as abovedescribed in connection with the higher frequency generator M or may bemodified as desired. The band-pass filter 12 may be constructed to passonly a relatively narrow band of frequencies centered at 200 cycles persecond.

Each of these tubes 42, 52 is provided, additionally, with t o furthercontrol electrodes or grids. The second grid 54 of the tube 42 isconnected through a bias battery 56 to the first stage plate resistor 34of the amplifier K and the second grid 14 of the tube 82 is connectedthrough a bias battery IE to the second stage plat resistor 36 of thesame amplifier. From these connections it will be apparent that thetubes 42 and 62 are partly controlled by the current to the principalscanning anode 22 of the dissector tube l2, such control being inopposite phase with respect to the tubes 42 and 82, so that a current tothe principal scanning anode, if it serves to raise the potential of thesecond grid 54 of the tube 42 and therefore tends to increase thetransconductance of that tube, serves equally to lower the potential ofthe second grid 14 of the tube 62 and thus drive it below its platecurrent cut-oil.

The third grids 55, 15 of the tubes 42 and 62 are similarly connectedthrough bias batteries 51, 11 to the first and second stage plateresistors 35, 31 of the amplifier J. Thus the tubes 42 and arecontrolled in part by the electron current which strikes the auxiliaryanode 23, such control being of opposite phase with respect to the twotubes 42 and 62 in the same manner as above described in connection withthe control from the rincipal scanning anode 22.

The grid bias batteries 56, 51, 16, ll of the tubes 42 and 62 areselected to hold tube 62 well above its cut-ofi' and to hold tube 42well below its cut-oil, so that a signal on either of the grids l4, 15of the tube 62 allows it to continue to conduct whereas signals on bothbrids 54, 55 of the tube 42 are necessary before that tube becomesconductive. The reason for these differences in bias potentials willbecome apparent hereinafter.

In addition to supplying its low frequency 05- cillations through thetube 62 and the band-pass filter 12 under control of signals from themain and auxiliary scanning anodes 22, 23 to line I as abov described,the low frequency generator 6i also supplies its oscillations to anotherline 80, which may be referred to as "line II." For this purpose itsterminals are similarly connected to the input circuit of a dischargedevice or vacuum tube 52. The anode 88 of this tube 82 is connectedthrough a loading resistor 89 and an anode supply battery to the cathode84, and its output terminals are connected through a stopping condenser2| to a narrow band-pass filter 92 which in turn is connected to lineII. This tub 82 is provided with one additional control electrode orgrid 04 which is connected directly to the second grid 14 of the tube62. It will be apparent from this connection that the tube 82 isrendered con:

ductive or non-conductive, as far as the second grids and therefore thprincipal scanning aperture signals are concerned, in phase with tube62, so that when signals from the low frequency 05- cillator ii aredelivered to line I they are delivered at the same time through the tube82 to line II.

Thus there are supplied either oscillations of 2000 cycles from thefirst oscillator 4| to line I or oscillations of a frequency of 200cycles from the second oscillator 6| either to line I alone or to bothline I and line II.

As above stated, the electron beam I! of the dissector tube l2 may beswept past the main and auxiliary apertures 20, 2! at speeds which varywith the tone values of the copy i being scanned. Further, as abovestated, this result may conveniently be attained through the use of afrequency-modulated signal. A circuit arrangement for securing thisresult, which may be substantially identical with the circuitarrangement fully to this portion of Fig. 1, a primary winding MI isdirectly connected across line I and to this winding IOI are coupled twosecondary windings I02, I03 which are poled 180 degrees apart andseparately connected to the input terminals of two discharge devicesI04, I05 which are biased below their cut-off points. The output circuitof the discharge device I04 includes a condenser I08 and a directcurrent source, for example a battery I01, the positive battery terminalalso being connected to one terminal I08 of a second condenser N of acapacitance many times as large as that of the first condenser I06. Theanode of the first discharge device I04 is directly connected to thecathode of the other device I whose anode is in turn directly connectedto the opposite terminal I09 of the large condenser I I0.

The deflecting elements of the image dissector tube, if they were platesfor electrostatic deflection, might be connected directly across theterminals of this larger condenser. Since in the modification shownmagnetic deflection is contemplated, the terminals of the largecondenser IIO are connected to the input circuit of a discharge device,for example a vacuum tube II5 of high input impedance whose output platecurrent is a replica of the input voltage wave form. The outputterminals of this tube II5 are directly connected to the beam-deflectingcoils I8 so that deflection of the beam I9 of the dissector tube I2takes place in accordance with the voltage across the large condenser II0.

Another discharge device H1 is connected across the terminals of thelarge condenser H0 in a manner to serve as a short-circuiting switch.For this purpose its cathode II 8 is connected to one terminal of thecondenser H0 and its anode H9 is connected through an anode battery I20and a relay coil I2I to the other terminal of the condenser I10. Thecontrol electrode I22 is connected through a high resistance I23 to thecathode I I8 and through a large variable biasing battery I24 to theother terminal of the condenser I I0. This bias battery I24 ispreferably chosen so that, as a charge builds up on the large con- 7 H1is preferably of the grid-controlled gas discharge type w ll known perse.

The circuit bove described constitutes a convenient form of cyclecounter whose operation will now bedescribed. Since the input windingsI02, I03 of discharge tubes I04, I05 are poled 180 degrees apart andtheir control electrodes biased below cut-off, these discharge devicesoperate in alternation. On a positive peak of voltage in the primarywinding I M, for example, the first tube I04 is rendered conductive sothat the small condenser I06 is charged to the voltage of the batteryI01, the second tube I03 mainwhile remaining non-conductive. On the nextnegative peak, say, of the signal, the first tube I04 is nonconductiveand the second tube I05 is rendered conductive so that the smallcondenser I06 is discharged into the large condenser IIO. Thus the stepby step, one step for each cycle of the signal in the primary windingIOI, independently of the frequency with which these half cycles recur.As

above stated, the voltage of the large condenser charge on the largecondenser H0 is built up H0 is impressed on the input circuit of thedischarge tube I|5 whose output current supplies the beam-deflectingwindings I8 of the dissector tube I2. Thus, the electron beam I9 of thedissector tub I2 is deflected in small steps, one step for each cycle ofthe signal in line I, the steps being of substantially equal magnitudeand recurring at the frequency of that one of the generators 4|, 5|which is instantaneously controlling and which, as explained above, isrelated to the tone values of the picture being scanned. Thus thescanning speed itself is related to the picture- 'tone values and, inthe example given, may b ten times as great for white areas as for blackareas. This process continues until the voltage of the large condenser II0 has reached a point such that the gas discharge tube H1 is no longerbiased below cut-off. This tube II1 then becomes conductive, effectivelyshort-circuiting the large condenser IIO and allowing the electron beamof the dissector tube I2 to return to its starting point, whereupon thecycle may commence once more. At the same time the pulse of currentthrough the tube II1 energizes the relay I2I which moves an armature I25against the tension of a spring I26 to draw the pawl 6 through thedistance of one tooth of the ratchet 5 and so advance the copy-bearingbelt 2 by the width of one scanning line of the copy I.

The ratio of the capacitanc-es of the condensers I06, I I0 is preferablysuch that each single step of the beam deflection is about 0.01 inch,which is approximately the length of an elemental area of a line of thecopy to be transmitted. Perfect equality in the magnitude of successivevoltage increments is obtained in the ideal case in which the codnenserI06 is negligibly small in comparison with the condenser IIO. In actualpractice the departure from linearity is less than 5 per cent when thelarge condenser is 10,000 or more times as great as the small condenserand this slight departure is substantially harmless. If

desired, however, a network having an inverse characteristic may beutilized to compensate for the residual non-linearity of the cyclecounting circuit. Various suitable networks of this kind will accur tothose skilled in the art.

Fig. 3 shows the circuit of a receiver printer system adapted to printdirectly from the signals delivered by the apparatus of Fig. 1.Referring to Fig. 3, the terminals of a primary winding 20I aredesignated line I to indicate that the signals delivered to it are theline I signals produced by the transmitter apparatus as above described.Coupled to this primary winding 20I are two secondary windings 202, 203which supply the input circuits of two discharge devices 204, 205associated with which are a small condenser 206 and a large condenser2I0. This portion of the receiver apparatus constitutes a cycle counterand the polarities of the windings 202, 203 and the connections of thisportion of the receiver circuit may be identical with those of the cyclecounter circuit described above in connection with the transmitter. Atripping tube 2I1 is connected across the terminals of the largecondenser 2I0 in the same manner and to the same eifect as thetransmitter tripping tube I I1.

A suitable printer tube for use in accordance with the invention may bethe tube described and claimed in Patent No. 2,273,433 granted to E.

Bruce Oct. 30, 1941, on an application flied April 10, 1940. Such a tubeis schematically shown in Hg. 8. It may comprise an evacuated glassenvelope in through the beam-receiving end of which protrudes a comb inthe form of a plurality of small discrete conductors ill arranged in asingle line. preferably 1Y1!!! in a plane at an oblique angle to thedirection of the scanning electron beam. The tube is provided with acathode iii, a beam-modulating grid ill, a beamdelining anode in havingan aperture i" therein which is preferably of rectangular form andconnected to a source of potential ill somewhat above that of thecathode, an accelerating and focusing anode schematically indicated byan apertured plate iSI connected through a resistor iii to a source 259oi relatively high potential and a pair of beam-deflecting elements, forexample electrostatic deflecting plates i". The interior walls oi thebeam-receiving end oi the tube are preferably lined with a conductivecoatingill, for example a coating of the material commonly known asaquadag, and this conductive lining is connected to the acceleratinganode i". A conductor in the form of a knife edge i is placed with itsedge closely adjacent the outer ends of the wires of the comb iii andconnected to the high potential source iii. The path of an electron beamit! originating at the cathode ili andatriking the comb wires illisindicated by a dotted line.

A strip or tape of blank sensitized paper illl passes close to or incontact with the outer ends of the comb wires ill and between them andthe knife edge it. It may be mounted on rolls one of which is providedwith a ratchet and pawl mechanism 5, 8 for intermittent advance thereof.just as in the case of the copy to be scanned at the transmitter. Thedeflecting elements in of this printer tube are connected to theterminals of the large condenser ill, a biasing battery iii beinginterposed to assure that the starting point of the cathode beam il'lshall be not at the center of the comb iil but to one side of it.

Conductors designated line H are connected through any suitablerectifier, for example a double triode i or an equivalent pair of simpletriodes, to the cathode iii and modulating grid ill. respectively, oithis printer tube, the grid being returned to the cathode in the usualmanner through a biasing battery iii and a resistor i. The voltage thebattery iii is preferably such that in the absence of signals on line Ithe printer tube is biased below cut-oil.

As the cathode beam of this printer tube is deiiected by the incrementalvoltage on the deflecting elements 2" and sweeps over the wires of thecomb iil, current passes out through successive wires, through thesensitized paper in to the knife edge i" and leaves a succession ofmarks on the paper in of character depending on the sensitizationthereof. This occurs only when the modulating grid iii is above cut-oi!potential, and therefore only when a signal is received on .line 11 toprovide modulating potential tor the aso'maa copy, at which time nomodulating signal is received on line II. As a result, the beam skipsrapidly over the light parts of the reproduction where no printing is tobe done with a considerable increase in over-all speed and correspondingsaving in time.

As above stated a slight inequality of th charging steps of thetransmitter condenser II. is harmless. This is due to the fact that thecharging of the receiver condenser illl remains in step with thecharging of the transmitter condenser so that any residual non-linearitydoes not result in geometrical distortion but only in a printed densityslightly greater at the terminating end of each line than at thestarting end.

When the cathode beam 261 of the printer tube has reached the end of itsline, that is, has undergone a deflection corresponding to a certainpreassigned voltage on the large condenser III, the discharge tube illoperates both to short-circuit the condenser i l O to return the cathodebeam il'l toits starting point and to energize the relay iil whichoperates the ratchet and pawl mechanism iii, iii to advance thesensitized tape in by the width of a single scanning line. ProperadJustment of the apparatus above described will sufilce to keeptransmitter and receiver in step or substantially so over periods which,while short, may be suilicient for the complete transmission of smallcopy. In case it is desired to maintain perfect synchronism over longerperiods, additional synchronizing signals may be resorted to. Suchsynchronizing signals may be of any desired character and may betransmitted in any convenient manner, for example over an additionalchannel which, however, may be as narrow as either 01' the channels 0!the invention.

The operation of the system as a whole will now be understood. In orderto avoid circumlocution the incidence of'electrons of the dissectorcathode beam ii on one or other of the two dissector apertures 20, iifrom a particular image point corresponding to a particular pictureelement of the copy I will be referred to in visual language by thestatement that the aperture "sees that particular picture element.

Assume now that the part of the copy I which both of the aperturestogether see is entirely white. Under these circumstances both theprincipal anode ii and the auxiliary anode 23 will carry full current,the first stages of the ampliners K and J will both be biased belowtheir cutoif points so that the potentials of the first stage loadresistors 34, "are raised and the potentials of the second stage loadresistors 36, 31 correspondingly reduced. This operates to raise thepotentials of the second grid 54 and the third grid ll of the tube Iiallowing the tube 42 to become conductive and deliver 2000-cycleoscillations from the generator 4| to line I. At the same time thisoperates to reduce the potentials of the second and third grids H, 15 ofthe tube 62 thus driving the tube Ii further below cut-off so that novoltage oi the 200-cycle signal from the generator ll is delivered toline I. At the same time the additional grid 04 of the tube ii which isdirectly connected to the second grid ll of the tube ii is reduced inpotential so that no signal at all is delivered to line II. Under theseconditions the cycle counter at the transmitter will cause scanning bythe electron beam l! of the dissector tube to take place at high speed.At the receiver too the cycle counter actuated by the 2000-cycle signalreceiver on line I causes the printer tube beam 261 to travel at highspeed while, since no signal appears on line II, the control grid 253 ofthe printer tube is biased below cut-oil? and no printing takes place.

Assume next. that as scanning at the transmitblack portions and it isdesirable that scanning at the receiver be slowed down to a speed suchthat the black portions can be printed with good definition and intheirproper location. This result is secured, in accordance with theinvention, by the fact that as soon as the auxiliary scanner aperture 2|sees some black areas the current,

through the auxiliary anode 23 is reduced, thus producing a rise in thepotential of the grid of the first stage of amplifier J past its cut-offpoint and a corresponding drop in the potential of the first plate ofamplifier J and rise in the potential of the second plate. Thesepotential changes, due to the connections of first and second stage loadresistors 35, 31, respectively of the amplifier J to the third grids 55,I of the tubes 42 and 52, operate to bias the tube 42 below cut-ofi thusremoving 2000-cycle signal from line I while at the same time permittingthe tube 62 to become conductive thus placing 200-cycle signal voltageon line I. The tube 82 in this event continues to be non-conductivesince its only additional grid 94 is connected to the second grid 14 oftube 52 which, as above stated, is under these conditions at a reducedpotential. Therefore, no printing signal appears on line II whereas, bythe substitution of 200-cycle signal for 2000-cycle signal on line I,the scanning speed is reduced at both transmitter and receiver.

, As a third condition, assume that scanning has advanced to the pointwhere the main scanner sees black areas while the auxiliary scannercontinues to see some black areas. Following the connections and thegrid voltage changes in the same manner as above, it will be seen thatunder these conditions the tube 42 is held below cut-ofi thus blockingthe 2000-cycle oscillator 4| while the tube 52 continues conductive thuspermitting 200-cycle signal from the oscillator 5| to continue to appearon line I. This causes the scanning operation, at both transmitter andreceiver, to progress at the slow speed. At the same time the additionalgrid 94 of the tube 82 which is connected to the second grid 14 of tube62 has been raised in potential. The tube 82, therefore, becomesconductive and ZOO-cycle voltage is-delivered from the oscillator 6|to'line II to operate the beam-modulating grid 253 of the printer tubeat the receiver station and cause the cathode beam 261 of the latter topass out of the tube through the comb wires 25| and leave impressions onthe sensitized paper 210 corresponding to black areas of the copy beingtransmitted.

As a last condition, assume that the main scanning aperture 2|! seesblack areas but that white areas are ahead so that the auxiliaryaperture 2| sees only white. In'such case it will be seen by followingthe connections and voltage changes in the same manner as abovedescribed that the -2000-cycle oscillator 4| continues to be blocked bynon-conductance of the tube 42 while the voltage of the ZOO-cycleoscillator 6| is delivered to line I through the tube 62 and also toline II through the tube 82. Therefore, due to the low frequency signalon line -I scanning continues to take place at a slow rate, and due tothe printing signal on line II, the cathode beam tively slow response.

261 of the printer tube continues to form impressions on the tape 210. M

Not until the original condition first described recurs, that is to saynot until both the main aperture 20 and the auxiliary aperture 2| seewhite together, will the printing signal on line II and the lowfrequency scanning signal on line I be blocked and the high frequencyscanning signal from the 2000-cycle oscillator 4| come into play oncemore to operate the scanning process at the higher speed correspondingto white areas.

The choice of different grid bias voltages for the tubes 42 and 62 willnow be understood. It is desirable in accordance with the inventionthatwhen either one ofv the two scanning apertures 20, 2| sees black,scanning shall progress atthe slow rate. Slow rate scanning, with thecircuits shown, means that the tube 42 is below its cutoff while thetube 62 is conductive. When one or other of theapertures 20, 2| seesblack, one or other of the additional rids 54, 55 of the tube 42 israised in potential and one or other ofthe additional grids 14, 15 ofthe tube 52 is reduced in potential. Since, as above stated, it isdesirable that under these conditions the tube 62 conduct while the'tube42 remains non-conductive, it is preferable that the tube 42 beinitially biased below its cut-off point and the tube 62 be initiallybiased above its cut-offpoint.

Thereason for the large size of the auxiliary aperture 2| as comparedwith the main scanning aperture 2|) will now be appreciated. As aboveindicated, in the case of ideal apparatus which responds instantaneouslyto an-impressed signal, no advance aperture is required at all. Suchinstantaneous response, in the case of transmission of an electricsignal from one point to another implies a transmission channel ofinfinite extent. Short of the ideal, response of a variable speedscanning system to a signal derived from the principalscanning aperture20 itself requires a transmission channelof very considerable extent;and, generally, the narrower the channel, the slower the response.

Therefore, in order to utilize a narrow transmission channel one must becontent with rela- (Slow response to a particular transition is hereintended. This is to be distinguished from slow reproduction of a groupof signals constituting a message as a whole.) In order that thenecessarily slow response of the narrow transmission channels of thesystem of the invention shall produce reproduction at the receiverstation which is of acceptable character, a delay may be introducedbetween the speed change signal and the printing signal, which delayvwill allow the scanning beam speed change to take full effect beforetheprinting commences; and this delay is inserted, in accordance withthe invention, by the use of the auxiliary advance scanner.

If, however, the advance scanning aperture were of the same size as theprincipal scanning aperture then, though it could see a. picture elementfar ahead of the principal scanner, it still could not operate correctlyunless further assisted by additional discriminating apparatus. Suppose,for example, that a portion ofthepicture being scanned consists of a rowof black dots spaced one picture element apart. Then the signal from theadvance scanner will change each time a boundary is crossed between ablack dot and its white background and the auxiliary scanher willdeliver a series of speed change signals with such rapidity that theapparatus cannot,

when it includes a narrow transmission channel of the charactercontemplated, distinguish between them at all, and the result will beutter confusion of the printing at the receiver. But with an auxiliaryscanner several times as large as the principal scanner as describedabove, the scanning apparatus at both transmitter and receiver may bearranged to maintain their slow scanning speed throughout such an areaof copy, and will rise to the high white scanning speed only when theauxiliary scanner sees white areas so far ahead that the scanning cansafely be accelerated, travel at high speed, and in turn be deceleratedin time to print the next black area correctly.

As stated above, the aperture of the auxiliary scanner is preferablyequivalent to some fifteen to twenty picture elements in length ascompared with the principal scanner aperture. This choice is based on ascanning speed ratio of ten to one plus a margin, for safety, of five toten picture elements in addition. Thus when the picture element seen bythe principal aperture changes from white to black the principal anodecurrent undergoes a full range change; but when a single black pictureelement enters the field of view of the auxiliary scanner aperturewhich, for example, has included only white areas at the previousinstant, the auxiliary anode current undergoes a change of the order ofonly per cent. This relatively small change should operate, inaccordance with the invention, to bias the tube 42 below cut-off andrender the tube 62 conductive. This result is accomplished with thecircuit arrangement of Fig. 1 by the selection of the bias potential ofthe amplifier J at such a value that the first stage remainsnon-conductive until the auxiliary anode current has increased to about95 per cent of its full white" value, becoming conductive only for fullwhite current. Thus the amplifier J is enabled to discriminate withprecision between the condition in which the auxiliary scanner aperturesees only white areas and the condition in which there are one or moreblack picture elements in its field of view.

It so happens that much of the copy which is normally transmittedcommercially, for example, typewritten material, contains on the averageseveral times as much white area as black area and very considerableportions of unrelieved white. By this invention, therefore, the scanningaparatus at both transmitter and receiver is enabled to speed up at suchunrelieved white portions thereby saving considerable time while stilloperating slowly and therefore with exactness in regions of greatdetail. Moreover, this is accomplished, in accordance with theinvention, with a very great saving in frequency space; that is, by theuse of the three channels above described, two narrow channels on line Iand a single narrow channel on line II.

In accordance with a further modification of the invention still furthereconomies in frequency space may be effected, the two channels of line Ibeing completely eliminated. This may be accomplished by transmittingthe sole black signal, storing it temporarily at the receiver, and inturn recovering it from the storage device at two difierent points bythe use of two separate pick-up devices. The signal from the principalpick-up device may operate the printer of the receiver tube in the samemanner as does the signal arriving on line II in the modification abovedescribed. The signal derived from the auxiliary pick-up device, whichis in advance of the principal pick-up device with respect to thestorage device and which is preferably of a somewhat larger aperture"than the principal pick-up device, serves to operate the scanningmechanism at the receiver in the same manner as does the signal arrivingover line I in the modification above described.

The transmitter apparatus embodying this modification may be identicalwith the apparatus of Fig. 1 above described with the sole exceptionthat in operation the signals of line I are not transmitted. They are,of course, utilized at the transmitter to effect variable speed scanningas described above.

Fig. 4 shows receiver apparatus embodying this modification of theinvention. Referring to Fig. 4, there is provided a storage device ofappropriate type, for example, a tape 3M of magnetizable material, whichmay be driven at constant speed by any appropriate mechanism, not shown.In its uniform progress the tape passes first between poles 302energized by a coil 30! carrying a direct current from any suitablesource 304. This coil "3 serves to remove all traces of previousmagnetization of the tape 3M and put it in condition to receive andretain records of the type for which it is intended.

The sole black signals reaching the receiver over line II may beamplified and converted into currents of related wave form in anyconvenient manner. Apparatus for this purpose is well known per se andis schematically indicated by the block 305 in Fig. 4. These sole blacksignal currents pass through a winding 306 to energize pole-pieces 301,which are also coupled to a biasing winding 308, through which thesignals are impressed magnetically on the tape Sill, The tape thenprogresses past the pole-pieces 309 of an auxiliary magnetic pick-updevice and after an interval past pole-pieces 3 i ll of a main pick-updevice, after which it has fully served its purpose and may be returnedto the demagnetizing coil 303 for reuse. With the exception of theauxiliary pick-up device which will be described below, this apparatusis well known per se and further details will not be given. It will beobvious to those skilled in the art that many other types of storagedevice may be employed in accordance with the invention, For example,the storage device may be a strip of fluorescent material and therecorder and reproducer appropriate optical or photoelectric units asdescribed in an article which appears in volume 27 of the Proceedings ofthe Institute of Radio Engineers for December 1939 at page 747. Again,the storage may be effected photographically on a photosensitive film inaccordance with the well-known technique of sound moving pictures. Ifpreferred, the storage device may be a wax record, cut by anelectromechanical stylus and reproduced from by an ordinary phonographpick-up device. Indeed, a storage device of any description may beemployed in accordance with the invention, though the magnetic tape iswell suited to secure the desired results.

The terminals of the principal pick-up coil ii! are connected to theinput terminals of a rectifier 3 here shown for the sake of simplicityas the well-known bridge connected arrangement of four dry platerectifier units. It is to be understood that any desired type ofrectifier may be employed.

Similarly, the auxiliary pick-up coil 3 supplies an auxiliary rectifierill which may likewise be of any desired type.

As in the case of the first modification, discrimination between blacksignals and white signalus on the frequency basis, though not essentialto the invention, constitutes a convenience and lends itself readily toincorporation as a part of the preferred embodiment. The secondmodification will therefore be described in conjunction with a frequencymodulated scanning signal similar to that of the first modification.Thus a high frequency generator 3, for example, of 2000 cycles persecond, supplies its voltage between the cathode 344 and first controlgrid 345 of a discharge tube 342 and a low frequency generator 36l, forexample of 200 cycles per second, supplies its voltage between cathodeand first control grid of another discharge tube 362. Each of thesetubes is provided with two additional control grids.

The output terminals of the principal rectifier 3 are directly connectedto the third grids 355, 315 of tubes 342 and 352, respectively; and theoutput terminals of the auxiliary rectifiers 3l3 are directly connectedto the second grids 354, 314 of the same tubes. A grid leak andcondenser pair are connected between each of the auxiliary grids of eachof the tubes 342 and 382 and the cathode of that tube. These grid leaksand condensers are included principally to smoothout the wave shapesobtained from the two rectifiers and may be replaced by any othersuitable apparatus for this purpose. The output circuits of the tubes342 and 362 are connected in a manner well known per se and supply theenergy of the 2000 cycle generator 34l through a band-pass filter 352and the energy of the 200 cycle generator 35! through a band-pass filter312 to the primary winding I of a cycle counter circuit which may beidentical with that described above in connection with Figs. 1 and 3 andmay operate in a similar manner to defiect the cathode beam 461 of aprinter tube 450,- which again may be identical with that of Fig. 3,both for scanning and for return of the printer tube beam 261 to itsstarting point,and to advance the sensitized paper tape410intermittently by the width of a single line of copy.

In addition, the beam-modulating grid 453 of the printer tube 450 isdirectly connected to the third grid of the tube 362 and thereforefollows the potential variations of the rectifier 3l4 associated withthe principal pick-up device 3H] and operates to extinguish the cathodebeam of the printer tube 450 when no signal is being picked up by theprincipal pick-up deviceand to permit the cathode beam 451 to impinge onthe comb l and form impressions on the sensitized tape 410 when blacksignals ar being picked up by the principal pick-up device.

The operation of the circuit above described is closely analogous tothat of Fig. 1 which was fully described above. It will be apparent bytracing the connections that when either the principal pick-up scanneror the advance pick-up scanner or both scanners see a black signal onthe magnetized tape 30! the 2000 cycle oscillator 3 will be blocked bythe tube 342 and the 200 cycle oscillator 36I will'deliver its voltagethrough the tube 362' and the band-pass filter 312 to the cycle countingcircuit and cause the cathode beam of the printer tube 450 to progressat the slow speed appropriate for dark areas. Only when both thescanning pick-ups together see that the tape 30! is clear of signals isthe 200 cycle oscillator 36] blocked and the 2000 cycle oscillator 34!permitted to deliver its signals through the band-pass filter 352 to thecycle counter circuit and cause scanning of the cathode beam of theprinter tube 450 to take place at the higher speed appropriate for whiteareas. Lastly, only when the principal scanner 3H] sees black does thecathode beam of the printer tube 450 reach the beam receiving comb 45!and produce impressions on the sensitized tape 410, being blocked underall other conditions by the potential of the beam modulating grid 453.

The aperture of the auxiliary pick-up device 309 may be some five to tenpicture elements in length corresponding to the length of fifteen totwenty elements for the auxiliary aperture at the transmitter. Thedifference is due to the fact that the receiver storage device, unlikethe transmitter scanner, progresses at constant speed, so that thescannig speed ratio component which in part determines the transmitterauxiliary aperture size, takes no part in the corresponding sizedetermination for the pick-up device at the receiver.

In general, the aperture size for a storage device of any type ismeasured by the size of that element which sees the record. For example,in the case of an optical storage device the aperture is measured by thesize of a light gate. In the particular case of the magnetic recordingdevice described above, the aperture is measured by the size of thepole-pieces. Depending on the oscillation freqeuncies chosen a singlepicture element may occupy the space of a fraction of a wave1ength onthe tape or of several wavelengths. Moreover, a continuous black portionof the copy may occupy any number of picture elements. Under theseconditions the portion of the magnetized tape 30| which is includedbetween the auxiliary pole-pieces 300 may include a number ofwave-lengths which is the product of the number of elements seen" by thepolepieces by the number of tape wave-lengths per element. For example,with auxiliary pole-pieces five elements in length, if one elementcorresponds to five tape wave-lengths this product would be twenty-fivewave-lengths.

In order that the auxiliary pick-up device may respond correctly to thepresence or absence of sole black signals on the tape its pole-piecesmay be serrated to provide oppositely spaced teeth so that the blackoscillator frequency is resonated. The same construction may be appliedto the main pick-up pole-pieces 3H], if they are more than onewave-length in extent. This construction is illustrated in Fig. 5, inwhich the dots 30l on the tape 30l represent individual wavelengths ofmagnetization, and each group of dots 30i is intended to represent ablack picture element, here taken by way of example as threewave-lengths long. In the case of the main polepieces 3I0, however, itis not necessary that they be more than one-half of the tape signalwavelength in extent, whether this be equal to a picture element lengthor not.

Band-pass filters 352 and 312 in the output circuits of the tubes 342and 362 do not serve the same purpose as the corresponding filters atthe transmitter, namely, to economize frequency space in transmission.They are included in the circuit of Fig. 4 rather to insure that thewave form of the frequency modulated signal impressed on the cyclecounter circuit shall be the same at the receiver as at the transmitter,so that scanning may take place similarly at the two stations. If thesefilters were not included, some distortion of wave form might occur dueto secondary effects of the tape 3M, the rectifiers Ill, 8 and othercircuit elements and this might result in difi'erences of scanningperformance at the transmitter and receiver.

Each of the circuit arrangements above described includes as a partthereof a cycle counter of a particular type. It will be understood thatany appropriate instrumentality may be used to convert the frequencymodulated signal into variable speed scanning of the cathode beam bothat transmitter and at receiver.

In both modifications described above the oscillation frequencies havebeen selected as 2000 cycles per second corresponding to white pictureareas and 200 cycles per second corresponding to black picture areas.These frequencies are taken by way of example only, and others wouldserve equally well, it being preferable, however, in order to make useof the advantages offered by the cycle counter circuit that thefrequency ratio be equal to the scanning speed ratio. If some other formof demodulating device were to be substituted for the cycle counter,this frequency ratio might be correspondingly altered in which case itwould be necessary only that the "white" oscillation frequency and the"black oscillation frequency differ by enough to enable the apparatus todiscriminate readily between them.

The preferred embodiments of the invention have been described in bothmodifications in conjunction with a frequency modulated signal ofpeculiar form and in conjunction with apparatus for generating such asignal and responding thereto. It must not be concluded that theinvention is limited in its application to frequency modulated signals,which constitute a convenience rather than an essential feature. Signalsof any type may be employed as long as they have a characteristic whichconstitutes a basis for discrimination between dark areas and lightareas of the picture to be scanned. Appropriate apparatus for generatingand receiving such signals and for responding thereto may if desired besub stituted, with appropriate modification, in the system of theinvention without in any way departing from the spirit thereof.Furthermore, it will be apparent to those skilled in the art that thecircuit arrangements above described may be modified to transmit andreconstitute pictures of more than two tones. Indeed, various featuresand advantages of the invention may be employed in related signalingarts, since though it is conceived that its chief use will be in thepicture signaling art and has been described in terms of illustrativeexamples suited to that art, it is not exclusively limited thereto.

It is to be understood that in the appended claims, in the absence ofspecific limitations, the word aperture" is used to designate whateversets the limit of the area of the field which is effective at anyinstant in the scanning operation. The term picture in the claims isintended to be a generic term and not to limit the claims with respectto the type of field which is analyzed or synthesized. Likewise, theterms "black" and white are used to designate two distinctive tones orcolors.

What is claimed is:

1. Signaling apparatus which comprises a principal scanner having anaperture bearing a stipulated relation to the length of a single recordelement and disposed to scan a record along a stipulated path, and anauxiliary scanner having an aperture bearing a similar relation to thelengths of a plurality of record lements and disposed to scan saidrecord along said path in advance of said principal scanner, means forderiving principal signals from said principal scanner, means forderiving auxiliary signals from said auxiliary scanner, and means undercontrol of said auxiliary signals for modifying the signals derived fromsaid principal scanner.

2. Apparatus for deriving picture signals from copy to be transmitted,which comprises a principal scanner having an aperture bearingastipulated relation to the length of a single elemental area of saidcopy and disposed to scan a line of said copy along a stipulated path,and an auxiliary scanner having an aperture bearing a similar relationto the length of a plurality of elemental areas of said copy anddisposed to scan said lin along saidpath in advance of said principalscanner, means for deriving principal signals from said principalscanner, means for deriving auxiliary signals from said auxiliaryscanner, and means under control of said auxiliary signals for modifyingthe signals derived from said principal scanner.

3. Picture signaling apparatus which comprises, a principal scannerarranged to scan a line of a sheet of copy to be transmitted, means forderiving principal signals from said principal scanner, an auxiliaryscanner having an aperture related in size to a plurality of pictureelements and arranged to scan said line in advance of said principalscanner aperture, means for deriving signals from said auxiliary scannerrelated to the tone values of a plurality of picture elements seen bysaid auxiliary scanner aperture, and means for regulating the scanningspeed of said scanners in accordance with said principal signals andsaid auxiliary signals together.

4. Picture signaling apparatus which comprises, a principal scannerhaving an aperture arranged to scan a line of a sheet of copy to betransmitted and related in size to a single picture element, anauxiliary scanner having an aperture similarly related in size to aplurality of picture elements and arranged to scan said line in advanceof said principal scanner aperture, means for deriving signals from saidauxiliary scanner related to the tone value of any one of a plurality ofpicture elements seen together by said auxiliary aperture, and means forregulating the scanning speed in accordance with said signals.

5. Picture signaling apparatus which comprises, a principal scannerhaving an aperture arranged to scan a line of a sheet of copy to betransmitted and of a size related to the size of a single pictureelement, an auxiliary scanner having an aperture similarly related insize to a plurality of picture elements and arranged to scan mid line inadvance of said principal scanner aperture, means for deriving signalsfrom said auxiliary scanner related to the tone values f pictureelements seen by said auxiliary aperture, means for utilizing saidauxiliary scanner signals, and bias means for causing said signalutilizing means to discriminate between the condition in which saidauxiliary aperture sees all white picture elements and the condition inwhich it sees one or more black picture elements, while notdiscriminating between the condition in which said auxiliary aperturesees one black element and the condition in which it sees more than oneblack element.

6. Picture signaling apparatus which comprises, a principal, scannerhaving an aperture for accelerating arranged to scan a line of a sheetof copy to be transmitted and of a size related in size to a singlepicture element, an auxiliary scanner having an aperture similarlyrelated in size to a plurality of picture elements and disposed to scansaid line in advance of said principal scanner aperture, means forderiving principal signals from said principal scanner, related to thetone values of successive single picture elements seen by said principalaperture, means for deriving auxiliary signals from said auxiliaryscanner related to the tone values of successive pluralities of pictureelements seen by said auxiliary aperture, means for regulating thescanning speed for both of said scanners in accordance with saidprincipal signals and said auxiliary signals, means for transmittingsaid principal signals to a receiver station, means for separatelytransmitting said auxiliary signals to a receiver station, and means atsaid receiving station for cooperatively utilizing said principal andauxiliary signals to reconstitute said copy.

7. In a picture signaling system, the combination which comprises, linescanning means for producing a principal signal related to the tonevalue of an individual picture element of a field to be transmitted, andmeans for producing auxiliary signals related to the tone values of aplurality of picture elements lying in advance of said first-namedelement in a scanning line, the first picture element of said pluralitybeing separated from said first-named individual picture element by asingle picture element, said plurality containing substantially fewerpicture elements than are contained in scanning line.

8. In the picture signaling art, the method which comprises scanningsuccessive elements of a picture to be transmitted at a certain instant,deriving from the elements scanned principal picture signals related tothe light and shade values of said elements, simultaneously scanning aplurality of other elements in advance of said first-named elements,deriving from said other elements auxiliary picture signals related tothe light and shade values of said other elements, regulating the speedof scanning in accordance with said principal signals and said auxiliarysignals, transmitting said principal signals and said auxiliary signalsto a receiving station, and at said receiving station, scanning animpressionreceiving element at speeds related to said principal andauxiliary signals, and printing said element in accordance with saidprincipal signals.

9. In a picture transmission system, the combination which comprises asheet of copy to be transmitted, a principal scanner disposed to scan aline of said copy, an auxiliary scanner disposed to scan said line inadvance of said principal scanner, means for deriving principal signalsfrom said principal scanner, means for deriving auxiliary signals fromsaid auxiliary scanner, means for accelerating and decelerating thescanning of said line in accordance with the signals derived from bothof said scanners, means for transmitting said principal signals to areceiving station, meansi'or receiving and recording said principalsignals in a storage device at said receiving station, means forrecovering signals from two separated points of said storage device, ascanner printer and an impressionreceiving element at said receivingstation, means and decelerating said scanning printer in accordance withthe signals reproduced the Whole of said at said two separated points,and means for supplied thereto,

causing said printer to impress said receiving element in accordancewith the signals reproduced at the second of said points.

10, Apparatus for reconstituting a black and white picture from a soletone signal received from a distant point, which comprises means forreceiving said signal, means for storing said signalat one instant,means for recovering said signal from said storage means at twbsuccessive later instants, a scanning printer, an impressionreceivingmedium, means for causing said printer to scan said receiving medium ata speed proportional to the frequency. of electric oscillations a firstgenerator of lower frequency oscillations and a second generator ofhigher frequency oscillations connected to supply scanning oscillationsto said scanning means, means controlled by the signals recovered atsaid two successive later instants for activating said first generatorand disabling'said second generator when one of said signals is presentand for activating said second generator and disabling said firstgenerator when neither of said signals is present, and means controlledby the signal later recovered for causing said scanning element toimpress said receiving medium when said later recovered signal ispresent and to be disabled when said later recov red signal is absent.

11. Apparatus for reconstituting a black and white image by successiveline scans from a sole tone signal received from a distant point, whichcomprises means for receiving said signal, a scanning element, animpressionereceiving medium, and means controlled by said receivedsignal for sweeping said scanning element over said receiving medium ata definitely determined speed in one direction during a line scan whensaid signal is received and at a different speed in the same directionduring said line scan when said signal is not received.

12. Receiver station apparatus for lineby-line reconstitution of a blackand white picture from sole tone signals received from a distant point.which comprises an impression-receiving medium, a printing device, meansfor causing said printing device to scan said receiving medium along a.line at a speed determined by the fre quency of electric oscillationssupplied thereto, two generators located at said receiver stationconnected to supply scanning speed determining oscillations to saidscanning means, means for disabling one of said generators upon thereception of said signals and the other of said generators in theabsence of said signals, and means for returning said printing device toits starting point at the conclusion of said line scan.

13. In the picture signaling art, the method which comprises scanningsuccessive elements of a picture to be transmitted, deriving from theelements scanned a signal of one frequency corresponding to pictureareas of one characteristic and a signal of a diil'erent frequencycorresponding to picture areas of a different characteristic,maintaining receiving apparatus at a receiving station in synchronismwith said signals, transmitting one of said signals through atransmission channel to said receiving station while sup pressing theother of said signals in said channel,

and at said receiving station, recreating areas of one of saidcharacteristics in accordance with said received signal and areas of theother of said characteristics under local control in part independent ofsaid signal.

14. Picture'signaling apparatus which comprises, a principal scannerhaving an a erture 'quency oscillations, a second generator of lowerfrequency oscillations, means for passing oscillations of said firstgenerator to a circuit when neither of said apertures sees black pictureelements and for passing oscillations of said second generator to saidcircuit when either or both of said apertures sees black picture areas,said passed oscillations constituting scanning speed signals, and meansfor regulating the scanning speed said scanner in accordance with saidscanning speed signals.

15. Picture signaling apparatus which comprises a principal scannerarranged to scan a line of a sheet of copy to be transmitted, means forderiving principal signals from said principal scanner, an auxiliaryscanner having an aperture related in size to a plurality of pictureelements and arranged to scan said line in advance of said principalscanner, means for deriving auxiliary signals from said auxiliaryscanner related to the tone values of picture elements seen by saidauxiliary scanner, a first generator of oscillations of a characteristicrelated to white image signals, a second generator of oscillations of adiflerent characteristic related to black image signals, means forpassing oscillations of said generators to a circuit alternatively inaccordance with said auxiliarysignals, and means for regulating thescanning speed of said scanners in accordance with said principal andauxiliary signals.

16. In combination with variable speed line-byline scanning apparatusfor reconstituting an image from received picture signals, storagemeans, means for forming a single track record of image signals on saidstorage means at one instant,.principal pick-up means for recoveringsaid signals from said single track record at a later instant" toprovide printing signals, and auxiliary picbup means for recovering saidsignals from said record at anintermediate instant to provide scanningspeed signals.

17. In picture signaling apparatus, the combination which comprisessignal storage means,

means iorforming a single track record of, image signals on said storagemeans, means for recovering said image signals from said single trackrecord at two separate later instants, and separate means fordifferently utilizing the signals recovered at said separate instants.

l8. Signaling apparatus'which comprises, a

of diflerentirequencies, means for selectively generators is actuated,and means for modulating said cathode beam under control 01 a particularone of said generators.

v20. In an electro-optical image transmission system, means for scanninga field to set up an image current any instantaneous value of whichcorresponds to the light tone value of an elemental field area, meansfor prescanning said field .along the elemental scanning pathsubsequently scanned by said first-mentioned scanning means, eachelement of said path instantaneously viewed by said prescanning means.having a length much greater than the corresponding dimension 0! anelemen instantaneously viewed by said first-mentioned scanning means,and means under control of said prescanning means for controlling therate of scanning by said first-mentioned scanning means.

21. Signaling apparatus which comprises variable speed means forscanning a portion of a field to be transmitted to derive principalsignals therefrom, variable speed means for simultaneously scanninganother portion of the same field to derive auxiliary signals therefrom,and means for varying the speed of said scanning means under the jointcontrol of said principal and said auxiliary signals.

22. The method of scanning to set up an image current which comprisesscanning the field along a chosen elemental stri path thereof, utilizinglight received from the field as the result 01' said scanning to causethe value of a characteristic of a current to accord at each instantwith the light-tone value of an elemental area of said path which iseffective at that instant, maintaining a certain constant velocity ofscanning while said characteristic of the resulting current is at aconstant value corresponding to a certain tone value present in saidfield, and changing the velocity of scanning only when a point isreached defining the beginning along said path of a portion thereo! fromwhich said tone value is. absent, the length of said portion along saidpath being large compared with the corresponding dimension of saidinstantaneously effective elemental area, but maintaining saidfirst-named scanning speed over path portions from which saidfirst-named tone value is absent, which portions are of lengthcomparable with said instantaneously efifective elemental area.

. 23. Apparatus for synthesizing a two-tone image from a signalconsisting of portions having a common distinguishing characteristicrepresentative of one of said tones and intervening blank portionsindicative of the other of said tones which comprises a scanningelement, an impression-receiving medium, means tor selectively sweepingsaid scanning element over said medium at different speeds correspondingto said different tones under control of said blank portions and saiddistinguishable portions of said signal, respectively.

24. In an image signaling system, the combination with a variable speedmeans for scanning elemental areas of a field in succession to produce aprincipal image signal representative of the tone values of saidelemental areas, of variable speed means for scanning other elementalareas of said field together as a group lyingin advance of a respectiveone of said firstnamed areas to produce an auxiliary signal dependentupon tone values of said group, and means tor varying the speed of saidscanning means under control 01 said auxiliary signal.

25. In an image signaling means, the combination of variable speed meansfor scanning successive areas of a field of view to be transmitted toderive image signals, means for causing said scanning means to travel ata comparatively low speed in scanning areas of' one light characteristicand at a comparatively high speed in scanning areas of a different lightcharacteristic, said speed-controlling means including means forinitiating acceleration only when an area scanned is of said first-namedlight characteristic and at least a stipulated plurality of similarareas of said second-named light characteristic' lie in advance thereof.

26. In an image signaling system, the combination with variable speedmeans for scanning a field to derive image signals, of means for causingsaid scanning means to travel at a comparatively low speed in scanningareas of one light characteristic independently of the extent of saidareas and at a comparatively high speed in scanning areas of a difierentlight characteristic of extent in excess of a stipulated amount.

27. An image tosignal translating device which comprises a photoelectriccathode extended in the direction of a line of an image formed thereon,two separate diflerently-apertured electron-receiving anodes spacedapart on a line parallel with said first-named line, the spacing of saidanodes and the aperture of one of said anodes being of the order of oneelemental image area in length, the aperture of the other of said anodesbeing of the order of a plurality of elemental image areas in length,means for directing electrons emitted from various points of saidcathode in a beam of substantial width in the direction of said line toimpinge on said anodes, means for deflecting said beam in the line ofsaid anodes in a manner to permit electrons of said beam originating ata particular image point to impinge first upon said larger anode andsubsequently upon said smaller anode, and means for deriving distinctand different signals from said separate anodes.

28. An image signaling system which comprises a principal scanner havingan aperture arranged to scan a line of an image to be transmitted, anauxiliary scanner having an aperture substantially larger than saidprincipal scanner aperture and arranged to scan said line in advance ofsaid principal scanner, means for deriving principal signals from saidprincipal scanner related to a light characteristic of areas seen singlyby said principal scanner, means for deriving auxiliary signals fromsaid auxiliary scanner related to a light characteristic of areas seentogether by said auxiliary scanner, and means controlled by saidauxiliary scanner signals for causing said principal scanner to passcomparatively rapidly over areas of one light characteristic andcomparatively slowly over areas of another light characteristic.

PIERRE MERTZ.

